I have found Aircon to make virtually no difference to consumption. I have just journeyed home 280 miles all motorway and dual carriageway.
Just after setting off I switched aircon on, it would have been unbearable without, I had tried for a few minutes.

It immediately took 10 miles off the GOM.
As I arrived back in Leeds and into traffic I chose to have the windows open and turned off the Aircon, it gave me back 4 miles, so cost of aircon over 280 miles was 6 miles on the GOM and journey completed at 4.3m/kWh.
I had done the outward journey, identical route but in reverse with no Aircon as weather was cooler and car was fine without it and achieved the same m/kWh.
NB it must make a difference somewhere after all you get nothing for free but I'm not noticing anything dramatic or that makes me think twice about putting it on when needed.